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Gastropods
 
 
 
 
 
The most common gastropod that we see today, and the one most easily recognized, is the snail.  Many a gardener knows what a snail looks like, and views it as a nemesis, since all land-dwelling gastropods are herbivores feeding on plant material.   To the gardener, the snail is an unwanted guest,  as it likes to dine on the gardenerís garden too!  Likewise many marine gastropods are also herbivores, although some are carnivores, preying on echinoids, crustaceans, and other molluscs such as bivalves.  To learn more about bivalves and echinoids, just click on the appropriate icons below.
Carnivorous gastropods appear to have first evolved during the Mesozoic era.  The first gastropods were exclusively marine, such as the Aldanella, Helcionella and Scenella which are found in rocks of the early Cambrian period.  Quite commonly fossil gastropods from the rocks of the early Paleozoic era are too poorly preserved for accurate identification.  One of the earliest known terrestrial, meaning land-dwelling, gastropods is Maturipupa.  It  is found in the Coal Measures of the Carboniferous period in Europe.  Fossil gastropods are less common during the Paleozoic era than bivalves.
 
 
Gastropods are a large and successful group of molluscs.  They have a worldwide distribution and an estimated 105,000 living species.  There are around 20,000 fossil forms.  First appearing early in the Cambrian period, some 570 million years ago, they have evolved to exploit a wide variety of marine, freshwater and land habitats.  Some modern marine species live at depths of more than 5,300 meters (17,400 feet), and on land both terrestrial and fresh-water gastropods have been found at elevations of up to 5,480 meters (18,000 feet).  Gastropods are the only molluscs to have adapted to life on land.  Other molluscs include bivalves and the cephalopods - ammonites, belemnites, nautiloids and goniatites.  Gastropod fossils may sometimes be confused with ammonites or other shelled cephalopods.  An example of this is Bellerophon from the limestones of the Carboniferous period in Europe which may be mistaken for a cephalopod.  The shells of ammonites may be superficially similar in appearance to those of flattened gastropods.  Ammonites however, and other cephalopods such as nautiloids and goniatites, are distinguished by a number of features including an internally chambered shell.  To learn more about cephalopods, click on the icon below. 
Gastropods have a continuous hollow shell.  As you can see in the cross-section below, though it might appear that the interior of the shell has subdivisions, the interior shell does not have any.  This is one of the major characteristics which differentiates the Gastropods from the Cephalopods, as with the Cephalopods, their shells have divided chambers.  The gastropod shell shows a great deal of variation in shape, but is usually coiled in a right-handed helical spiral. The surface of the shell may display growth lines, and can be either smooth or ornamented with crests, ribs, ridges and spines.
The term given to each 360 degree turn of the gastropod shell is the whorl.  The 
remaining turns of the shell are collectively called the spire of the shell. The last whorl, which is the largest in size, is called the body whorl.  The opening of the body whorl from which the creature emerges is called the aperture of the shell.  The largest fossil gastropod is Campanile giganteum, which also measures over 50 centimeters, and is often found in the rocks of the Eocene period in the Paris basin in France. 

In rocks of the Mesozoic era gastropods are slightly more common as fossils, their shells often are well preserved.  Their fossils occur in beds which were deposited in both freshwater and marine environments.  Rocks of the Cenozoic era yield very large numbers of gastropod fossils, many of these fossils being closely related to modern living forms.  The diversity of the gastropods increased markedly at the beginning of this era, along with that of the bivalves. 

Certain trail-like markings preserved in ancient sedimentary rocks are thought to have been made by gastropods crawling over the soft mud and sand. Although these trails are of debatable origin, some of them do resemble the trails made by living gastropods today.  Such fossils are called trace fossils, and to learn more about trace fossils, just click on the image below. 

To see what gastropod fossils we have found, just click on the icon below. 
 
 
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